How do social encounters produce changes in the brain? The long term goal of the proposed experiments is to identify anatomical pathways that external influences use to cause cell specific changes in the brain. We will continue studying the well defined social system of a teleost fish in which we have shown that changes in social status change the brain. We will manipulate the social and reproductive states of animals under controlled laboratory conditions that mimic their natural environment and measure the cellular and molecular consequences. We will focus on gonadotropin-releasing hormone neurons (GnRH) in the hypothalamicpreoptic area using cellular and molecular probes and a newly developed transgenic animal. When males in this species ascend in status, GnRH containing neurons enlarge by ca. 8-fold in volume and when they lose status, these same cells shrink. We have discovered that the dendritic arbors of these cells also change, increasing during ascent and decreasing during descent. This system offers numerous advantages for the analysis of the cellular consequences of social behavior. Since GnRH neurons are phylogenetically ancient, central to the control of reproduction, these studies will provide insight across vertebrate phyla. We will address several key questions about the cellular consequences of behavioral status: 1) Do the dendritic arbors of GnRH neurons change in response to social cues, enlarging and interconnecting in dominant males and shrinking and disconnecting in non-dominant males? Are there more synaptic connections in dominant than in non-dominant males? 2) What genes expressed in GnRH neurons of dominant males vs non-dominant males? 3) What brain nuclei are active in the transition between dominant and non-dominant states? 4) What are the anatomical connections to the GnRH neurons? 5) Are the neuronal firing patterns different between dominant and non-dominant males?